This invention concerns a method and apparatus for removing or stripping a layer of resist material from a substrate surface of different material, such as a semiconductor slice, in the fabrication of an electronic structure, such as a discrete semiconductor device including transistors, diodes, etc., an integrated circuit, or a printed circuit board, wherein the removal of the layer of resist material is accomplished by subjecting same to the flame from a combustible gas for a period of time sufficient to chemically decompose the resist material.
Photoresist materials are commonly used in a wide number of industrial processes where it is desired to provide detailed patterns on a substrate surface in thin films applied thereto or to provide regions of altered character as contrasted to other surface portions of the substrate. Such photoresist materials are photosensitive in nature, being characterized by differential reactivity to specific liquid solvents after exposure to any energy source, such as ultraviolet radiation, for example. A standard photographic practice in employing photoresist layers as patterns involves the application of a layer of photoresist material to a substrate, followed by the selective exposure of the photoresist layer to an energy source, wherein portions of the photoresist layer are changed in character due to their exposure to the energy source. After such exposure, the photoresist layer is then developed by a "wet development process" employing liquid chemical solvents to selectively remove portions of the photoresist for providing the desired pattern therein. Negative and positive photoresist materials are available for this purpose. In a negative photoresist material, the portion of a layer thereof exposed to the energy source is changed in its chemical character being polymerized as contrasted to the non-exposed portion and rendering it insoluble with respect to the liquid chemical solvent which removes the non-exposed portion of the negative photoresist layer in producing the desired pattern therein. For a positive photoresist layer, the situation is reversed in that the portion thereof exposed to the energy source is rendered soluble to the liquid solvent, while the non-exposed portion is insoluble with respect thereto and remains intact after development in forming the desired pattern.
Stripping of the patterned photoresist layer becomes necessary in most instances in the fabrication of an electronic structure after its purpose in providing a patterned mask has been served, such as for the selective application of a thin film to the underlying substrate surface or for the application of dopant materials to selected regions of the substrate, for example. Many fabrication processes for manufacturing discrete electronic devices and integrated circuits require repeated uses of photo-resist layers at various stages of the process before the electronic structure is completed. Removal or stripping of each photoresist layer after it has served its purpose in the fabrication of the electronic structure is required before the additional stages in the fabrication process may be accomplished, or at least as the final stage in completing the fabrication of the electronic structure.
Many different types of "wet" solvent developers have been employed to remove exposed or unexposed portions of a photoresist layer from an underlying substrate surface. Typical of such wet developers are the hydrogen peroxide-sulfuric acid developers and those based on phenol-methyl ethyl ketone and trichloroethylene. In the hydrogen peroxide-sulfuric acid system, a mixture of 10 to 33.3% of hydrogen peroxide and 66.6-90% of sulfuric acid can be used for the complete removal of the photoresist after acid etching on the exposed portion of the underlying substrate surface to be patterned has been completed. In general, such a system may be used for all photoresist removal processes except those requiring removal of a photoresist layer from an aluminum surface, since this particular system will dissolve aluminum. Inherent in the use of these chemicals is the problem of contamination from inorganic ions, which remain on the resist patterned substrate, commonly referred to as the "slice", after the development of the pattern is complete. This contamination results in additional chemical treatment being required of the developed slices for removal of such contamination involving further time, handling and expense in completing the fabrication of the electronic structure.
The class of wet developers based on phenol-methyl ethyl ketone and trichloroethylene is typically used on substrates having aluminum metalization layers, since this photoresist development and removal system does not chemically attack aluminum. However, development with this system is also time-consuming and is accompanied by a substantially high incidence of contamination.
Further, use of a wet solvent developer is generally responsible for the production of some residual material from the removed photoresist layer which remains on the substrate surface. This residual material also is present along the edges of the photoresist pattern where it tapers down to the substrate surface causing a decrease in pattern resolution tending to render the pattern detail less sharp in outline.
A further problem peculiar to the etching of an aluminum layer on a substrate, wherein the aluminum layer is initially overlaid by a photoresist layer which is developed by a conventional wet developer to provide an etch pattern for exposed portions of the aluminum layer, is the generation of hydrogen bubbles by the chemical etchant in reacting with the aluminum metal. These hydrogen bubbles tend to form on the surface of the aluminum layer, particularly seeming to congregate around the residue left by the wet developer. Their presence prevents fresh acid from reaching the aluminum surface and inhibits proper etching thereof. Previously, various wetting agents and mechanical agitation have been employed to alleviate this problem with some success in that these means have facilitated the release of the hydrogen bubbles from the aluminum surface.
In addition to wet process development techniques for removing a layer of photoresist material, another technique which may be said to be based on a non-solvent or dry basis has heretofore been employed for the purpose of removing photoresist material. This technique employs plasma etching as obtained through an oxygen RF plasma process wherein the generated plasma containing monatomic oxygen as an active species is employed to remove the layer of photoresist material. A description of plasma etching for photoresist removal may be found in the article "A Dry Photoresist Removal Method"--Irving, published in Kodak Photoresist Seminar Proceedings--1968, edition, Vol. II, pp. 26-29. Generally, plasma containing monatomic oxygen O may be generated by placing diatomic oxygen (O.sub.2) molecules in an evacuated chamber and providing an electric field of sufficient force to disassociate the oxygen (O.sub.2) molecules. The so-called plasma machines or "ashers" are generally AC systems operating at radio frequencies in the Khz range. Typically, two opposing horizontal flat electrode plates or tubular electrodes are employed for purposes of generating the plasma therebetween. As recognized in the art, the term "asher" is generally applied to an RF plasma machine employed for the purpose of removing photoresist material by plasma etching. This oxygen RF plasma technique, although effective to remove photoresist material, is characteristically slow, requiring from 10 to 30 minutes for removal of the photoresist material and also commonly requiring batch processing rather than continuous processing, wherein individual slices or groups of slices must be loaded and unloaded in a batch procedure from a Bell jar. Furthermore, semiconductor slices incorporating MOS technology (metal-oxide-semiconductor technology) may be subject to degradation by the use of this technique for removing photoresist material therefrom because of their excessive sensitivity to electric fields.
Another technique for stripping photoresist material from a substrate which may be said to be a so-called dry removal method employs an ozone treatment and is described in detail in pending application Ser. No. 601,861, filed Aug. 4, 1975, by Samuel Ray Shortes et al and assigned to the assignee of the present invention. The ozone treatment technique involves the exposure of the photoresist layer to an ozone-containing gaseous atmosphere in the reaction zone of a reactor, the ozone being present as an active reagent in the gaseous atmosphere to which the layer of photoresist material is exposed in an amount sufficient to react with all of the photoresist material in the layer thereof, with the photoresist material being removed from the underlying substrate surface in response to exposure to the ozone. Although the aforementioned ozone technique effectively eliminates most of the stated problems inherent in wet development techniques, it requires a special atmosphere and utilizes a gas which is both expensive and potentially dangerous to human beings in large amounts.